Why graph? Data in a table is hard to interpret. A good graph turns numbers into a picture that shows the trend instantly. Pick the right graph for your data, and label it properly.
Picking the right graph type
Match your data type to the graph that shows it best.
Bar graph
Use when: comparing categories
e.g. favourite colour of students in each grade.
Line graph
Use when: change over TIME
e.g. temperature each hour throughout a day.
Pie chart
Use when: parts of a whole
e.g. percentage of energy from each source (sun, wind, gas).
The 5 essentials every graph needs
- Title describing what is being measured.
- Labels on both axes, including UNITS (cm, °C, seconds).
- Scale that uses even intervals (don't jump from 0,1,2 to 5,10,20).
- Data points or bars drawn carefully and clearly.
- Legend if comparing more than one group.
Common pitfall: forgetting units on axes. "Time" is not a label. "Time (seconds)" is.
Convention: independent variable (what you changed) goes on the x-axis (horizontal). Dependent variable (what you measured) goes on the y-axis (vertical).
Reading a graph: what to look for
1. Trend
Is the line going up, down, or staying flat? That's the overall pattern.
2. Highs and lows
What's the highest point? Lowest? When did each happen?
3. Rate of change
A steep line = fast change. A flat line = no change. A line that gets steeper means change is speeding up.
4. Outliers
Any single point that's far away from the rest? That might be an error, or it might be the most interesting thing on the graph.
5. Compare two lines
Where do they cross? Where is one higher than the other? What does that tell you?
Worked example: ice cube melting
An experiment timed how many minutes it took an ice cube to fully melt in water at 4 different starting temperatures.
| Water temp (°C) | Time to melt (min) |
|---|---|
| 10 | 12 |
| 20 | 7 |
| 30 | 4 |
| 40 | 2 |
Right graph: line graph, because both variables are numerical and we want to see how time changes as temperature changes. x-axis: Water temp (°C), y-axis: Time to melt (min). Trend: as temperature rises, melting time drops fast (negative relationship).